Acoustic pulse generator utilizing a mechanism for changing the natural frequency of oscillation

ABSTRACT

An acoustic pulse generator for generating acoustic pulses of varying frequency in water is disclosed including a mechanically resonant structure, a drive mechanism for causing the mechanically resonant structure to oscillate, a mechanism for changing the natural frequency of oscillation of the mechanically resonant structure, and one or more transducers for transmitting the oscillations of the mechanically resonant structure to the water in which the acoustic pulse is to be generated.

United States Patent Graham et al.

ACOUSTIC PULSE GENERATOR UTILIZING A MECHANISM FOR CHANGING THE NATURALFREQUENCY OF OSCILLATION Inventors: Walton Graham, Roslyn; Irving E.Melnlck, Syosset; Tullio De Filippis, Garden City, all of NY.

Control Data Corporation, neapolis, Minn.

Filed: Nov. 21, 1969 Appl. No.: 878,776

Assignee: Min- U.S. Cl. ..340/8 R, 181/.5 H Int. Cl. ..G0lv 1/02 Fieldof Search ..340/8, 12; 181/.5 H

References Cited UNITED STATES PATENTS Lyons et al. ..l8l/.5 l-l 51Sept. 12, 1972 7/1968 Dickie et al ..340/8 X Primary Examiner-Carl D.Quarforth Assistant Examiner-J. M. Potenza Attorney-Darby & Darby [57]ABSTRACT An acoustic pulse generator for generating acoustic pulses ofvarying frequency in water is disclosed including a mechanicallyresonant structure, a drive mechanism for causing the mechanicallyresonant structure to oscillate, a mechanism for changing the naturalfrequency of oscillation of the mechanically resonant structure, and oneor more transducers for transmitting the oscillations of themechanically resonant structure to the water in which the acoustic pulseis to be generated.

29 Claims, 11 Drawing Figures PATENTEDSEP 12 1972 SHEET 1 [IF 5 FIG.

FIG. 2

POWER (WATTS) I,OO0,000-

PATENTEUSEP 12 m2 SHEET 2 OF 5 JOmFZOU moEEO PATENTEDSEPIZIHIZ 3.691.516

sum 3 0F 5 INVENTORS WALTON GRAHAM IRVING E. MELNICK TULLIO De FlLLlPlSAT TORNEYS PATENTEU 3,691,516

SHEEI '4 OF 5 ATTORNEYS ACOUSTIC PULSE GENERATOR UTILIZING A MECHANISMFOR CHANGING TI-IE NATURAL FREQUENCY OF OSCILLATION This inventionrelates generally to apparatus for generating acoustic pulses such asare used in geological exploration, and particularly in offshore oilexploration. The travel times of signals reflected by internal layers ofthe earth are used to calculate the position of such layers. In earlyexploration work, explosives were commonly used to generate acousticpulses. Explosives generate strong, sharp acoustic pulses having afairly wide frequency range. However, the use of explosives is notpermitted in certain areas if, for example, damage to structures or wildlife is likely. For this reason, attempts have been made to provide asafe, controlled acoustic pulse generator which is capable of spreadingthe requisite acoustic energy over a period of time.

Devices which are capable of transmitting a lowpower acoustic signal fora period of several seconds are generally well-known to those skilled inthe art. However, in order to obtain accurate geological information, itis necessary to be able to accurately measure the time between thetransmission of the acoustic signal and the reception of the signalreflected back from the various internal layers of the earth. Becausethe acoustic signal is transmitted over a period of several seconds, itis necessary to be able to associate with each instant of the period oftransmission some unique property of the transmitted signal which can beidentified in the reflected signal. Accordingly, such acoustic pulsegenerators used in geological exploration are capable of generating anacoustic signal having a frequency which varies with time. For example,a typical acoustic signal for use in underwater geological explorationmight have a duration of several seconds beginning at a frequency of hzand sweeping linearly up to 100 hz, or vice versa. The frequencies ofthe reflected signals can readily be identified and correlated with thefrequency of the transmitted signal with an accuracy equal to a smallpart of the total duration of the transmitted signal.

The useful acoustic power output of an underwater acoustic pulsegenerator is limited by the fact that cavitation will occur during thereduced-pressure portions of the acoustic wave if the strength of theacoustic signal is too great. However, the strength of the acousticsignal should be maximized to assure a strong reflection from thevarious layers of earth. Since the strength of the transmitted acousticpulse is limited by the cavitation phenomenon, the acoustic pulsegenerator should be lowered well below the water surface in order totake advantage of the higher threshold of cavitation at those depths. Inaddition, the reflection from the water surface of the transmittedsignal (in the 10 hz to 100 hz frequency range) can be made favorable byplacing the source at approximately 40 feet beneath the surface of thewater.

In addition, if the transducer is a simple piston in contact with thewater, the amplitude of the motion of the piston must be changed withthe frequency of the acoustic signal so as to keep the acoustic signalstrength at its maximum value just below the cavitation threshold.

Certain mechanisms for producing mechanical oscillations of variablefrequency and variable amplitude are known to those skilled in the art.For example, the

desired mechanical oscillations can be produced by a hydraulic servosystem including a hydraulic power supply, a hydraulic servo valve andan actuating piston. The hydraulic servo valve converts the statichydraulic pressure into the desired flow to the actuating piston. Thehydraulic servo valve is actuated by an electric solenoid, and controlof the mechanism is effected by impressing the required electric signalon the solenoid.

While hydraulic servo systems provide good efficiency when drivingdissipative loads, their overall efficiency drops to a low value whendriving a reactive load, such as water, particularly when the wavelengthof the acoustic signal is large in relation to the diameter of thetransducer piston.

For example, in a hydraulic system for generating acoustic waves inwater, the transducer might be an acoustic piston rigidly attached tothe hydraulic actuating piston. The acoustic piston would be ofsufficient diameter and rigidity to produce the maximum acoustic signalstrength throughout the range of frequencies of interest. The loading onthe acoustic piston is predominately reactive rather than dissipative.That is to say that the water acts as a large mass attached to theacoustic piston. In many oscillatory or reciprocating mechanicalsystems, a reactive load poses no great problem because the reactiveenergy oscillates between the power source and the load with only asmall internal dissipative loss. On the other hand, hydraulic servosystems are unable to transfer the energy from the load back through tothe power source. This deficiency arises from the nature of the servovalve which passes hydraulic fluid to the actuating piston to move theload, but only converts the reactive energy from the load into heat onthe return stroke.

It is therefore an object of this invention to provide improvedapparatus for generating controlled acoustic pulses in water.

It is also an object of this invention to provide a compactself-contained acoustic pulse generator which may be contained within asubmarine capsule of moderate size.

It is a further object of this invention to provide a self-containedacoustic pulse generator having an electric power input only.

It is another object of this invention to provide an acoustic pulsegenerator which is capable of generating acoustic pulses in water withhigh efficiency.

According to the above and other objects, the present invention providesan acoustic pulse generator including a mechanically resonant structure,a drive mechanism for causing the mechanically resonant structure tooscillate, a transducer connected to the resonant structure forconverting the oscillations of the mechanically resonant structure intoacoustic waves in a medium such as water, and a mechanism for changingthe natural frequency of oscillation of the mechanically resonantstructure and the stroke of the transducer in order to provide a longstroke at low frequencies and a short stroke at high frequencies. Inaddition, the preferred form of the present invention includes a cockingdevice for storing energy in the resonant structure to permit theacoustic output pulse to be initiated at a high power level. Sensingdevices and feedback control mechanisms are provided to control thesystem so that acoustic pressures generated by the piston do not exceedthe cavitational limit.

The above and other objects and advantages of the present invention willbe apparent to those skilled in the art from the following detaileddescription and accompanying drawings which set forth the principles ofthe invention and, by way of illustration, the preferred embodiment forcarrying out those principles and several modifications thereof.

In the drawings:

FIG. 1 is an overall view of a geological survey ship with a submarinecapsule containing the acoustical pulse generator of the presentinvention.

FIG. 2 is a graph of the factors limiting the power output of theacoustic pulse generator of the present invention over the range offrequencies of interest.

FIG. 3 is a side elevational view in cross-section taken along line 3-3of FIG. 6 of a submarine capsule containing a preferred form of acousticpulse generator according to the present invention.

FIG. 4 is a cross-sectional view of the submarine capsule and acousticpulse generator taken along the line 4-4 of FIG. 3.

FIG. 5 is a block diagram of the preferred form of feedback controlsystem for the drive mechanism of the subject acoustic pulse generator.

FIG. 6 is a plan view with parts broken away of the submarine capsuleand acoustic pulse generator.

FIG. 7 is a cross-sectional view of the submarine capsule and theacoustic pulse generator taken along the line 77 of FIG. 6.

FIG. 8 is a side elevational view in cross-section of a submarinecapsule containing a modified form of acoustic pulse generator accordingto the present in vention.

FIG. 9 is a crosssectional view of the submarine capsule and modifiedacoustic pulse generator taken along the line 99 of FIG. 8.

FIG. 10 is a perspective view, partly broken away, of a second modifiedform of acoustic pulse generator according to the present invention.

FIG. 11 is a perspective view, partly broken away, of a third modifiedform of acoustic pulse generator according to the present invention.

Referring now to FIG. 1 of the drawings, there is shown a generaloverall view of an offshore seismic prospecting operation using theacoustic pulse generator of the present invention. The survey ship 1moves over a prescribed course which is provided by various navigationaldevices not shown. A submarine capsule 2 is towed behind the ship 1preferably at a depth of 40 feet beneath the surface of the water.Electrical power is supplied via cable 4 to the acoustic pulse generatorcontained within the submarine capsule 2. At regular intervals, underthe control of the crew of the survey ship 1, the acoustic pulsegenerator is triggered and an acoustic pulse is transmitted to the waterb3acoustical pistons 5 and 6. The acoustic waves pass through the waterand on into the earth beneath, whence they are reflected from varioussubsurface layers. The reflected waves, or echoes, are received by thehydrophones 7 which are placed at internals along the streamer 8 whichis towed behind the survey ship 1. The signals from the hydrophone arerecorded, and later analyzed to determine the depths and locations ofthe various subsurface layers to provide a picture of the subsurfacestructure of the earth beneath the water. The details of the subsurfacestructure can then be analyzed to determine if the characteristics ofpetroleum-bearing strata or other valuable mineral deposits are present.

Referring now to FIG. 2 of the drawings, there is shown a graph of thefactors limiting the power output of an acoustic pulse generator of thetype shown in FIG. 1 over the range of frequencies of interest. Thereare three limits on the acoustic power output. There is a cavitationlimit (P, which is due to the properties of the medium (water) throughwhich the acoustic waves are to be transmitted. Attempts to exceed thecavitation limit will result in the production of cavitation bubblesrather than increased acoustic power output. At a depth of about 43feet, the power limitation imposed by the cavitation phenomenon is:

P (watts) 40,000[b(meters) In addition, if for mechanical engineeringreasons the velocity of the acoustic pistons should not exceed a certainmaximum value, v there will be a resulting limit, P,, on output power.P,, is given by:

P (watts) 41 [f(hz)] [v (meters/sec.)] [12 (meters) Similarly, if formechanical reasons, the displacement of the acoustic pistons should notexceed a certain maximum value d,,,,,, there will be a resulting limit,P on output power. P is given by:

P (watts) 1630 [f(hz)] [d (meters)] [d(meters) 14 These three limitingfactors on the acoustic output power are graphically shown in FIG. 2 fora typical acoustic pulse generator according to the present inventionwherein the radius of the piston, b 0.5 m, v 2 m/sec., d,,,,, 0.15 m.According to equations 1, 2 and 3 above, P 10,000 watts, P, 10f and P2.3 f. In the frequency range between l0hz and l00hz, P or P, are thelimiting factors on the acoustic power output. Referring now to FIG. 3of the drawings, there is shown a side elevation view in cross-sectionof a submarine capsule containing the preferred form of acoustic pulsegenerator according to the present invention. The submarine capsule,generally designated 20, is of oblong shape so that it may be relativelyeasily towed under water by a survey ship as shown in FIG. 1. The outershell of submarine capsule 20 may be made of steel or other suitablematerial and should be watertight so as to prevent the entry of seawaterinto the interior where the acoustic pulse generating apparatus islocated.

As shown in FIG. 3, a pair of acoustic pistons 22 and 23 are located atopposite ends of the submarine capsule 20. Piston 23 slides within acylinder 25. A circumferential sealing ring 26 surrounds piston 23 andcontacts the walls of cylinder 25 to prevent water from entering thecylinder 25 behind piston 23. Piston 22 slides within cylinder 28 and isprovided with a similar circumferential sealing ring.

Rigidly mounted at the center of piston 23 is a connecting rod or shaft29 which is connected, at its other end, to the beam member 31 whichforms a part of the variable frequency drive mechanism which will beexplained in greater detail hereinafter. A similar connecting rod orshaft 32 connects piston 22 to beam member 34. Connecting shaft 29slides longitudinally in bearings and 36. Similar bearings are providedfor connecting shaft 32. Connecting shafts 29 and 32 are preferablymounted in colinear relation so that the forces generated by pistons 22and 23 are balanced and there is no net force or torque acting uponsubmarine capsule 20.

Projecting inwardly from piston 23 is an annular structure orcylindrical wall 37, the inner surface of which engages the outersurface of fixed cylindrical projection 38. Annular structure 37, thecentral portion of the inner face of piston 23 and the end of fixedcylindrical projection 38 form a variable volume chamber which isconnected via conduit 41 and guide release valve 41a, to an air surgetank 42. Variable volume chamber 40 is also connected via conduit 24 andair pressure regulator and valve 27 to the main air reservoir 77. Piston22 is provided with a similar chamber, not shown, which is connected viaconduit 43 and quick release valve 43a to air surge tank 44 and, viaconduit 30 and air pressure regulator and valve 33 to the main airreservoir 77.

In operation the variable volume air chambers, such as chamber 40,perform two functions. They are used to cock the resonant structureprior to the generating of an output pulse so as to provide a highoutput power level at the start of the pulse, and they are used tobalance the static pressure of the water on the outer surfaces ofpistons 22 and 23.

For example, referring to variable volume chamber 40, the cocking of theresonant structure is accomplished by admitting air from reservoir 77via conduit 24 to chamber 40 under control of air pressure regulatorvalve 27. The air pressure which is used to cock the resonant structureis preferably sufficient to move the piston 23 to the outwardmost limitof its stroke acting against both the static pressure of the water onthe outer surface of piston 23 and against the restoring force of thepneumatic spring system which will be described in greater detailhereinafter. Normally, the air pressure which is required to cock theresonant structure is about twice that which is required to balance thestatic pressure of the water when the pistons 22 and 23 are centered attheir neutral positions.

When the pistons 22 and 23 have been moved to their outwardmostpositions, the air pressure regulator valves 27 and 33 are closed toplace the apparatus in condition to begin the generating of an acousticoutput pulse. The output pulse is started by opening quick releasevalves 41a and 43a to permit air to pass from the variable volumechambers thru the conduits 41 and 43 to air surge tanks 42 and 44 thussharply reducing the air pressure in the variable volume chambers to alevel which just balances the static force of the water on the outersurfaces of pistons 22 and 23. The sharp pressure reduction in thevariable volume chambers, such as chamber 40, causes the pistons 22 and23 to move inward thus initiating the acoustic output pulse.

The air surge tanks 42 and 44 have a volume which is sufficient toproduce the required air pressure reduction. The necessary volumedepends upon the pressure within the air surge tanks 42 and 44 beforethe start of the acoustic pulse. If the tanks 42 and 44 are evacuatedprior to the start of the pulse, each tank will have a volume which isapproximately equal to the mean volume of its associated variable volumechamber. lf, prior to the start of an acoustic pulse, the air surgetanks 42 and 44 contain air at a certain pressure, such as the ambientpressure within submarine capsule 20, each tank 42 and 44 must have asomewhat larger volume in order to produce the required pressurereduction in their associated variable volume chambers.

During the generating of an acoustic output pulse, the air surge tank 42provides additional volume so that the pressure within chamber 40 willremain more nearly constant as the size of chamber 40 changes with themotions of piston 23. Air surge tank 44 performs a similar function inconnection with the variable volume chamber associated with piston 22.

After an acoustic output pulse has been generated, quick release valves41a and 43a are closed and exhaust valves 42a and 44a are opened topermit air surge tanks 42 and 44 to be evacuated by the vacuum pump 47or, alternatively, to vent the air in tanks 42 and 44 into the interiorof submarine capsule 20. Air pressure regulator valves 27 and 33 may beopened when it is next desired to cock the system.

Surrounding annular structure 37 and fixed cylindrical projection 38 isan annular vacuum chamber 45. A similar vacuum chamber 46 is associatedwith piston 22. The vacuum chambers 45 and 46 are evacuated by a vacuumpump 47 via conduits 48 and 49 respectively. Chambers 45 and 46 areevacuated so as to avoid significant pressure changes arising from themotions of pistons 22 and 23 and to prevent leakage of air into waterwhen pressure on piston is minimum. This permits use of a relativelyloose seal around pistons 22 and 23 thus reducing friction loss.

It will be appreciated by those skilled in the art that the entire spacebehind each of the pistons 22 and 23 might be devoted to pressurebalancing chambers such as chamber 40. In that case, however, the airsurge tanks 42 and 44 would have to be considerably larger in order tomaintain a substantially constant pressure in the pressure balancingchambers throughout the range of motion of the pistons 22 and 23.

The vacuum pump removes both air and seawater which may have leaked intoannular chambers 45 and 46. The air and seawater are separated, and thesea water is pumped overboard via conduit 51 while the air is dischargedinside the hull of submarine capsule 20.

As described above, the connecting rod or shaft 29 of piston 23 isconnected to a beam member 31. One end of beam 31 is connected to apneumatic spring system which receives the reactive energy transmittedfrom the reactive load (seawater, in this case) back to the piston 23connecting shaft 29 and beam member 31. The other end of the member 31is connected to the drive mechanism which provides the motive force forthe piston 23 and which provides the desired variation in frequency andstroke.

More particularly, one end of beam member 31 is connected by a rod 53 toa piston 54 which slides within an air cylinder 55. The air cylinder 55is pivotally mounted on the frame of the submarine capsule 20 by a pivotpin 56, and the connecting rod 53 is pivotally mounted on beam member 31by a pivot pin 57 in order to permit free movement of these parts duringthe operation of the acoustic pulse generating apparatus of the presentinvention.

Similarly, beam member 34 is connected by a rod 63 to a piston 64 whichslides within an air cylinder 65. Air cylinder 65 is pivotally mountedon the frame of submarine capsule by a pivot pin 66, and connecting rod63 is pivotally mounted on the beam member 34 by a pivot pin 67.

The chamber 58, which is the portion of air cylinder 55 to the right ofpiston 54 as shown in FIG. 3, is connected by a relatively largediameter flexible pneumatic hose 59 to the air surge tank 70. Similarly,the chamber 68, which is the portion of air cylinder 65 to the left ofpiston 64 as shown in FIG. 3, is also connected by a relatively largediameter flexible pneumatic hose 69 to the air surge tank 70. Thechamber 72, which is the portion of air cylinder 55 to the left ofpiston 54 as shown in FIG. 3, is connected by the large diameterflexible pneumatic hose 73 to a second air surge tank 71 which is notshown in FIG. 3, but is shown in FIG. 5. The chamber 74, which is theportion of air cylinder 65 to the right of piston 64 as shown in FIG. 3,is connected by the large diameter flexible pneumatic hose 75 to the airsurge tank 71 as shown in FIG. 5.

The two air cylinders 55 and 65 and the two air surge tanks 70 and 71act as a pneumatic spring system for the acoustic pulse generatingapparatus of the present invention. When the acoustic pistons 22 and 23are in their neutral positions as shown in FIG. 3, the pistons 54 and 64are approximately centered in their respective air cylinders 55 and65and the air pressures in the two air surge tanks 70 and 71 are equalso that the forces on pistons 54 and 64 are balanced. When the acousticpistons 22 and 23 move outward, the volumes of chambers 58 and 68 arereduced and the pressure in chambers 58 and 68 and air surge tank 70 isincreased. At the same time, the volumes of chambers 72 and 74 areincreased and the pressure in chambers 72 and 74 and air surge tank 71is increased. As a result of these different pressures, there is arestoring force on pistons 54 and 64 which tends to return them to theirneutral positions shown in FIG. 3. Conversely, when the acoustic pistons22 and 23 move inward, the pressure in air surge tank 70 is reducedwhile the pressure in air surge tank 71 is increased. Again, thedifference in pressures provides a restoring force which tends torestore pistons 54 and 64 to their neutral positions shown in FIG. 3.Hence, the air cylinders 55 and 65 and the air surge tanks 70 and 71 actas a pneumatic spring system. The volumes of air cylinders 55 and 65 andair surge tanks 70 and 71 are preferably related to the diameter andstroke of pistons 54 and 64 so that the restoring force arising from theabove-mentioned pressure differences is proportional to the displacementof the pistons 54 and 64 from their neutral position shown in FIG. 3.There is a general limitation that fractional change in volume producedby the motions of pistons 54 and 64 should be small enough so that thenonlinearities will be acceptable. The actual dimensions and pressuresare selected to produce the required spring constant.

The air in air surge tank 70 and 71 is supplied from an air reservoir 77through suitable pressure regulators which are not shown in FIG. 3 andwhich may be of a type well-known to those skilled in the art. Air issupplied to the air reservoir 77 by a conduit'78 from an air compressor79 through an air supply regulation and pressure sensing device 80.Another conduit 81 from air supply regulation and pressure sensingdevice supplies high pressure tanks 82 and 83 shown in FIG. 4.

The drive mechanism includes a hydraulic actuator having output shafts91 and 92. The output shaft 91 is pivotally connected to the cross-slide93 which engages ball screw 94. The ball screw 94 is rotated by a motor95 to adjust the point at which beam member 31 is driven by hydraulicactuator 90. The forces generated by hydraulic actuator 90 arepreferably transmitted directly through cross-slides 93 to beam member31 rather than through the ball screw 94. If desired, roller bearingsmay be provided to facilitate the motion of cross-slide 93 along beammember 31 in response to the ball screw 94.

Similarly, the output shaft 92 of hydraulic actuator 90 is pivotallyconnected to a cross-slide 96 which engages a ball screw (not shown)which is driven by a motor 97. In operation, the two output shafts 91and 92 of hydraulic actuator 90 move in unison, which is to say thatboth shafts move outward at the same time and both move inward at thesame time.

The hydraulic actuator 90 is controlled by a servo valve 100 which isconnected to a hydraulic reservoir 101 and a hydraulic pump 102 which isdriven by a motor 103. The servo valve 100 is controlled by the feedbackcontrol system shown in block diagram form in FIG. 5. Feedback signalsare provided to the electronic control system by acceleration sensingdevices 105 and 106 mounted on piston shafts 29 and 32 respectively asshown in FIG. 3. The acceleration sensing devices 105 and 106 may beaccelerometers of the type well-known to those skilled in the art.

The acoustic pressure generated by pistons 22 and 23 is directlyproportional to their acceleration. Hence, the maximum accelerationsignal produced by acceleration sensing devices 105 and 106 gives ameasure of the maximum acoustic pressure generated by pistons 22 and 23during their outward stroke, and the minimum pressure which occursduring the return stroke. Because cavitation will occur if the pressureat the faces of pistons 22 and 23 drops below a certain minimum value onthe return stroke, it is desirable to control the acceleration of thepistons 22 and 23 in order to avoid cavitation.

In the preferred form of the acoustic pulse generator of the presentinvention, control of the maximum acceleration of the acoustic pistons22 and 23 is accomplished by the feedback control system of FIG. 5. Thetime-varying signals from the acceleration sensing device shown in FIG.5 are fed into peak-following circuit 151 which provides an outputsignal which corresponds to the peak value of the time-varying inputsignal. The output signal from peak-following circuit 151 is compared,in comparator 152, with a reference signal corresponding to theacceleration at which cavitation would occur at the particular depth atwhich the acoustic pulse generator is being operated. The output signalfrom comparator 152 corresponds to difference between the maximumacceleration signal provided by acceleration sensing device 150 andpeak-following circuit 151 and the reference signal and is fed to theservo valve orifice control 153 to control the size of the orifice ofthe servo valve 100. If the difierence is large, the orifice will beopened wide so as to provide more force to the output shafts 91 and 92of hydraulic actuator 90 thus increasing the maximum acceleration ofacoustic pistons 22 and 23. On the other hand, if the difference betweenthe reference signal and the maximum acceleration signals fromacceleration sensing devices 105 and 106 approaches zero, the orifice ofservo valve 100 will be restricted so as to limit the force applied byoutput shafts 91 and 92 of hydraulic actuator 90, thus limiting theacceleration of the acoustic pistons 22 and 23 to a value just belowthat which would produce cavitation. In operation, the effect of theabove-described feedback control system is to operate the acoustic pulsegenerator of the present invention so as to produce the maximum possibleacoustic output power as limited by the threshold at which cavitationeffects occur.

It will be appreciated that the reference signal is preferablyadjustable to correspond to the operating depth of the acoustic pulsegenerator of the present invention. The greater the depth, the greaterthe maximum acceleration which can be tolerated without producingcavitation effects.

In the preferred form of acoustic pulse generator shown in FIG. 3, thefrequency of operation is changed by changing the effective lengths ofbeam members 31 and 34. This is accomplished by operation of the screwjack, such as screw jack 94 of beam member 31, to

A change the positions of cross-slides 93 and 96. When the cross-slides93 and 96 are located at their maximum distances from piston shafts 29and 32 respectively, the acoustic waves produced by the motions ofpistons 22 and 23 will be at the low end of the frequency scale,nominally hz as suggested hereinabove. The reason for this is that thepneumatic spring system including air cylinders 55 and 65 and air surgetanks 70 and 71 sees a large load relative to its fixed spring constant.During the production of each acoustic pulse, the motors 95 and 97 drivethe ball screws to move crossslides 93 and 96 towards the centers ofbeam members 31 and 34 respectively. As the cross-slides 93 and 96approach the piston shafts 29 and 32 respectively, the pneumatic springsystem acquires a greater mechanical advantage and, hence, sees asmaller load relative to its fixed spring constant. Therefore, thenatural frequency of oscillation of the apparatus increases as thecross-slides 93 and 96 approach the points of attachment of pistonshafts 29 and 32 at the center of beam members 31 and 34 respectively.

From the foregoing it will be appreciated that each beam member 31 and34 acts as a second-class lever wherein the cross-slide is the fulcrum,the piston shaft is the load and the pneumatic spring system is theapplied force. It will be appreciated that the fulcrum is essentiallystationary because the motions of the hydraulic actuator are extremelysmall in relation to the motions of the acoustic pistons 22 and 23 andthe pistons 54 and 64 of the pneumatic spring system. For example, theacoustic pulse generator of the present invention would typically have aQ on the order of 100 at lOhz. Therefore, the stroke of the outputshafts 91 and 92 of hydraulic actuator 90 would be only one-fiftieth ofthe stroke of the acoustic pistons 22 and 23. Because of the shortstroke of the hydraulic actuator 90, losses in the hydraulic system areminimized so that, for a given acoustic output power level, the inputpower requirements of the system are relatively low.

In the preferred form of acoustic pulse generator according to thepresent invention, the frequency of operation of the hydraulic actuatoris changed to match the natural resonant frequency which changes withthe operation of the ball screws as described above. The frequency ofoperation of hydraulic actuator 90 is controlled by servo valve which isin turn controlled by the feedback control system which is shown in FIG.5. The signal which controls the frequency of operation of servo controlvalve 100 is derived from the time-varying output signals ofacceleration sensing device 150 which corresponds to either of theacceleration sensing devices and 106 shown in FIG. 3. The output signalsfrom acceleration sensing devices 105 and 106 are oscillating signalshaving the same frequency as the motions of the acoustic pistons 22 and23 which correspond to the natural resonant frequency of the system.Hence, the output signals from acceleration sensing device may be used,with appropriate phase modification by phase delay circuit 155, tocontrol the servo valve 100 so that the hydraulic actuator will continueto operate at the natural resonant frequency of the system as thatnatural frequency changes with the operation of the ball screws.

FIGS. 8 and 9 are cross-sectional views of a submarine capsule 200containing a modified form of acoustic pulse generator according to thepresent invention. The outer shell of the submarine capsule 200 ispreferably made of steel or other suitable material and has astreamlined oblong shape so that it may be relatively easily towed underwater by a survey ship as shown in FIG. 1. In addition to the acousticpulse generating apparatus which will be described in greater detailhereinafter, the submarine capsule 200 may contain various types ofauxiliary equipment such as, ballast tanks, pumps and valves, bilgepumps, etc.

As shown in FIG. 8, a pair of acoustic pistons 201 and 202 are locatedat opposite ends of the submarine capsule 200. Acoustic pistons 201 and202 are located within suitable cylindrical openings 203 and 204respectively in submarine capsule 200. Suitable circumferential sealingrings are provided in order to prevent seawater from leaking in aroundthe edges of pistons 201 and 202 into the interior of submarine capsule200.

The acoustic piston 201 is connected to the drive mechanism by aconnecting rod 205 which slides longitudinally within a bearing 207which is supported by the structure of submarine capsule 200. Similarly,the acoustic piston 202 is connected to the drive mechanism by aconnecting rod 206 which slides longitudinally within a bearing 208which is supported by the structure of the submarine capsule 200. Theaxes of travel of the two acoustic pistons 201 and 202 are colinear sothat the forces generated by the motions of the pistons are balanced andthere is no net force or torque on the submarine capsule 200 as a resultof the operation of the acoustic pulse generating apparatus.

The acoustic pulse generating apparatus of FIGS. 8 and 9 include anelastic beam 210 which is preferably made of steel or other materialcapable of efficiently storing substantial amounts of mechanical energywhen elastically distorted. The elastic beam 210 is supported by twomovable support structures 211 and 212. Each of the movable supportstructures 211 and 212 includes roller bearings 213 contacting the upperand lower surfaces of the elastic beam 210. The lower portions ofmovable support structures 211 and 212 are in the form of cross-slides215 and 216 which ride within ways 217 and 218 and engage ball screws219 and 220 respectively. The ball screws 219 and 220 are rotated by amotor 221 which operates to move both support structures 211 and 212simultaneously inward towards the center of beam 210, or to move bothsupport structures 211 and 212 simultaneously outward toward the ends ofthe beam 210. Changing the positions of movable support structures 211and 212 will change the natural resonant frequency of the elastic beam210. When the two movable support structures 211 and 212 are near thecenter of the elastic beam 210, the natural resonant frequency will berelatively lower, and the excursions of the ends of the beam 210 will berelatively longer. On the other hand, when the movable supportstructures 211 and 212 are near the outer ends of the elastic beam 210,the natural resonant frequency will be relatively higher and theexcursions of the ends of the beam 210 will be relatively shorter.Hence, the characteristics of the elastic beam 210 are well matched tothe requirements for operating the acoustic pistons 201 and 202 toprovide the maximum acoustic power output over the prescribed frequencyrange. As explained previously, the stroke of the acoustic pistonsshould be longer at lower frequencies and shorter at higher frequenciesin order to produce the maximum output power over the prescribedfrequency range without exceeding the cavitation limit.

The elastic beam 210 is connected to acoustic piston 201 by a link 223which is connected to a bell crank 225 which is connected to a link 227which is connected to the connecting rod 205 of acoustic piston 201.Similarly, elastic beam 210 is connected to acoustic piston 202 by alink 222 which is connected to bell crank 224 which is, in turn,connected to link 226.

The length and cross-sectional dimensions of the elastic beam 210 andthe proportions of the linkages connecting the elastic beam 210 to theacoustic pistons 201 and 202 are interrelated and depend upon the forcesand displacements to be applied to the acoustic pistons 201 and 202 inorder to achieve the prescribed acoustic output power over theprescribed frequency range.

The elastic beam 210 is driven by a pair of actuators 229 and 230 whichform portions of the movable supporting structures 211 and 212respectively. The actuators 229 and 230 are preferably hydraulicactuators which are controlled by a feedback control system similar tothat described in connection with the preferred embodiment shown inFIGS. 3-7. Briefly, the feedback control system includes accelerationsensing devices mounted, for example, on connecting rods S and 206. Thetime-varying signals from the acceleration sensing devices are used,with appropriate phase delays, to control the servo-valve which controlsthe actuators 229 and 230. The magnitude of the signals from theacceleration sensing devices is compared with a reference signal, andthe difference is used to control the force applied by the actuators 229and 230 in order to keep the acoustic output power of the apparatus at avalue just below the cavitation limit over the prescribed frequencyrange.

Although hydraulic actuators are preferred, it will be appreciated bythose skilled in the art that other types of actuators may be employedwithin the spirit and scope of the present invention, such, as, forexample, stacks of piezoelectric crystals. In any event, it will beappreciated that because of the large 0 of the system and because of thedisplacement multiplying effect of the linkages between the elastic beam210 and the acoustic pistons 201 and 202, the displacements of theactuators 229 and 230 need only be on the order of fractions of an inch.

Referring now to FIG. 10 of the drawings, there is shown a perspectiveview of another modified form of the acoustic pulse generating apparatusof the present invention. The apparatus of FIG. 10 includes an elasticbeam 300 which is preferably made of steel or other material capable ofefficiently storing mechanical energy when elastically distorted. In theacoustic pulse generating apparatus of FIG. 10, the elastic beam 300 isadapted to be twisted or torsionally distorted by the action of theactuator 301. One end of the elastic beam 300 is pivotally supported bya stationary supporting structure 302 which may be, for example, aportion of the hull or supporting structure of a submarine capsule ofthe type shown in FIG. 1 and FIGS.38. The other end of the elastic beam300 is rigidly connnected to a shaft 303 having a crank 304 which isoperated by the actuator 301. If desired, the shaft 303 may bejournalled in a portion of the stationary supporting structure 305.

The natural resonant frequency of the elastic beam 300 is controlled bythe position of the movable supporting structure 310 which includes setsof roller bearings 311 which contact the upper and lower surfaces of theelastic beam 300. The lower portion 312 of the movable supportingstructure 310 slides within a way 313 which is mounted on the stationarysupporting structure indicated by fragmentary portion 314. The lowerportion 312 of movable supporting structure 310 engages the ball screw315 which is operated by the motor 316. The lower portion 312 of movablesupporting structure 310 preferably fits within the way 313 so thatforces on the movable supporting structure 310 are transmitted directlyto the way 313 and not to the ball screw 315.

It will be appreciated by those skilled in the art that the naturalresonant frequency of the elastic beam 300 will be lowest when themovable supporting structure 310 is farthest from the end of the elasticbeam 300 which is driven by the actuator 301. Conversely, the resonantfrequency will be highest when the movable supporting structure 310 isclosest to the end driven by the actuator 301. Moreover, as in the caseof the elastic beam 210 of FIGS. 8 and 9 which was subjected tolongitudinal bending, the rotational motions of the driven end of theelastic beam 300 of FIG. 9 are greatest when the frequency ofoscillation is low, and are shortest when the frequency of oscillationis high.

The motions of the driven end of elastic beam 300 are transmitted to theacoustic piston 320 by the crank arm 321 which is connected to link 322which is in turn connected to the connecting rod 323 of piston 320.

The connecting rod 323 slides longitudinally in an appropriate bearing324 in the supporting structure 325. Similarly, a crank arm 331 which ismounted on the driven end of elastic beam 300 is connected to a link 332which is in turn connected to the connecting rod 333 of acoustic piston330. The connecting rod 333 slides longitudinally in a suitable bearing334 in supporting structure 335.

As in the case of the acoustic pulse generating apparatus of FIGS. 8 and9, the length and cross-sectional dimensions of the elastic beam 300 andthe proportions of the connecting linkages of the acoustic pulsegenerating apparatus of FIG. 10 are determined by the forces anddisplacements which are to be applied to the acoustic pistons 320 and330 over the intended frequency range.

The actuator 301 is preferably a hydraulic actuator controlled by afeedback control system of the type described in connection with theprevious embodiments shown in FIGS. 3-9. It will be appreciated,however, that other types of actuators and control systems may be usedwithin the spirit and scope of the present invention.

FIG. 11 shows a perspective view of yet another modified form of theacoustic pulse generator of the present invention. The elastic beam 400is preferably made of steel or other material capable of efficientlystoring mechanical energy when elastically distorted. The naturalfrequency of oscillation of the elastic beam 400 is controlled by themovable supporting structures 401 and 402 which include sets of rollerbearings 403 which engage the upper and lower surfaces of the elasticbeam 400. The lower portions 405 and 406 of the movable supportingstructures 401 and 402 respectively slide within a way 407 which ismounted on stationary supporting structures 408 which may be portions ofthe hull or internal structure of a submarine capsule of the type shownin FIG. 1 and FIGS. 3-9. The lower portions 405 and 406 of the movablesupporting structures 401 and 402 engage ball screws 409 and 409a whichare driven by motors 410 and 410a, respectively. The ball screws 409 and409a operate to move the two movable supporting structures 401 and 402simultaneously inward toward the center of the elastic beam 400, orsimultaneously outward toward the ends of the elastic beam 400. Thenatural resonant frequency is the lowest, and the excursions of the endsof the elastic beam 400 are greatest when the movable supportingstructures 401 and 402 are positioned near the center of the beam.Conversely, the resonant frequency is highest, and the excursions of theends of the elastic beam 400 are shortest when the movable supportingstructures 401 and 402 are positioned near the ends of the beam 400.

The motions of the end s of elastic beam 400 are transmitted to theacoustic pistons 411 and 412 by a connecting linkage including links 413and 414 which connect the ends of the elastic beam 400 to the crank arms415 and 416 which project from the shaft 417 which is preferablyjournalled at both ends in the stationary supporting structure asrepresented by fragments 418 and 419. The motions of the ends of elasticbeam 400 thus cause the shaft 417 to oscillate, in a rotational sense,about its longitudinal axis. The crank arm 421, which is mounted onshaft 417, transmits the motions of shafts 417 to link 422 which isconnected to lever 423 which is, in turn, connected to link 424 which isconnected to the connecting rod 425 of acoustic piston 411. Theconnecting rod 425 slides longitudinal ly within a bearing 426 which ismounted in the supporting structure 427. Similarly, the crank 421transmits the motions of shaft 417 to link 432 which is connected to theleverarm 433 which is, in turn, connected to the link 434 which isconnected to the connecting rod 435 of acoustic piston 412. Theconnecting rod 435 slides longitudinally within the bearing 436 which ismounted in supporting structure 437.

The elastic beam 400 is preferably driven at its center by a hydraulicactuator 440 which is controlled by a feedback control system of thetype described in connection with the embodiments shown in FIGS. 2-10.It will be appreciated, however, that other types of actuators may beused within the spirit and scope of the present invention. It willfurther be appreciated that the elastic beam 400 might be actuatedalternatively by two actuators forming parts of the movable supportingstructures 401 and 402 as described above in connection with theembodiment of FIGS. 8 and 9.

It will further be apparent to those skilled in the art that othermodifications and adaptations of the present acoustic pulse generatingapparatus may be made without departing from the spirit and scope of theinvention as set forth with particularity in the appended claims.

What is claimed is:

1. An acoustic pulse generator for generating acoustic pulses in amedium which acts as a reactive load, each acoustic pulse comprising anacoustic wave having a frequency which varies over a predetermined rangeduring the period of said pulse, said acoustic pulse generatorcomprising:

a mechanically resonant structure capable of storing mechanical energyin the form of oscillatory movements; transducer connected to saidmechanically resonant structure, said transducer being in contact withsaid medium to convert the oscillatory movements of said mechanicallyresonant structure into acoustic waves in said medium;

drive means for imparting sufficient energy to said mechanicallyresonant structure to produce an acoustic pulse; and

means for changing the natural of oscillatory of said mechanicallyresonant structure over a predetermined range during the period of anacoustic pulse.

2. The acoustic pulse generator of claim 1 wherein said drive meanscomprises an actuator for imparting oscillatory motions to saidmechanically resonant structure.

3. The acoustic pulse generator of claim 2 wherein said transducercomprises a piston connected to said mechanically resonant structure,the face of said piston being in contact with said medium to convert theoscillatory movements of said mechanically resonant structure intoacoustic waves in said medium.

4. The acoustic pulse generator of claim 3 further comprising meansresponsive to the motions of said system for controlling the frequencyof operation of said actuator.

5. The acoustic pulse generator of claim 1 wherein said mechanicallyresonant structure comprises a pneumatic spring system including an aircylinder, a piston disposed within said air cylinder so that when saidpiston is displaced from a neutral position, a differential pressure iscreated which tends to return said piston to said neutral position.

6. The acoustic pulse generator of claim 5 further comprising a beampivotally connected to said piston of said pneumatic spring system sothat the action of said piston is substantially transverse to said beam,said transducer and said drive means being connected to said beam atdifferent points along the length thereof.

7. The acoustic pulse generator of claim 6 wherein said means forchanging the natural frequency of oscillatory of said mechanicallyresonant structure comprises means for changing the ratio of thedistance between the point of connection of said pneumatic spring systemto said beam and the point of connection of said drive means to saidbeam, and the distance between the point of connection of saidtransducer and said beam and the point of connection of said drive meansand said beam.

8. The acoustic pulse generator of claim 7 wherein said means forchanging the natural frequency of oscillatory of said mechanicallyresonant structure comprises a ball screw mounted on said beam along aportion of the length thereof, a motor for driving said ball screw, anda cross-slide engaging said ball screw, said drive means being connectedto said ball screw so that, upon operation of said motor, saidcross-slide is caused to move along the length of said beam, therebychanging the point of application of said drive means to said beam.

9. The acoustic pulse generator of claim 1 wherein said mechanicallyresonant structure comprises an elastic beam capable of storingmechanical energy when elastically deformed and releasing said energy inthe form of oscillatory movements.

10. The acoustic pulse generator of claim 9 wherein said transducer isconnected by a mechanical linkage to a free end of said elastic beam.

11. The acoustic pulse generator of claim 10 wherein said drive meanscomprises an actuator for imparting oscillatory motions to said elasticbeam.

12. The acoustic pulse generator of claim 11 wherein said means forchanging the frequency of oscillatory of said elastic beam comprises asupport member engaging said elastic beam, said support member beingstationary in a plane perpendicular to the longitudinal axis of saidelastic beam but movable along the length of said elastic beam to changethe natural frequency of oscillatory thereof.

13. The acoustic pulse generator of claim 11 wherein said actuatorengages said elastic beam at approximately right angles to thelongitudinal axis thereof so as to impart bending oscillations thereto.

14. The acoustic pulse generator of claim 11 wherein said actuatorengages a crank arm extending at right angles to the longitudinal axisof said elastic beam so as to impart torsional oscillatory to saidelastic beam.

15. Acoustic pulse generating apparatus for generating acoustic pulsesin water, each acoustic pulse comprising an acoustic wave having afrequency which varies over a predetermined range during the period ofsaid pulse, said acoustic pulse generating apparatus comprising:

a mechanically resonant structure capable of storing mechanical energyin the form of oscillatory movements;

a pair of acoustic pistons connected to said mechanically resonantstructure, the faces of said acoustic pistons being in contact with thewater to convert the oscillatory movements of said mechanically resonantstructure into acoustic waves in the water;

drive means for imparting sufficient energy to said mechanicallyresonant structure to produce an acoustic pulse; and

means for changing the natural frequency of oscillatory of saidmechanically resonant structure over a predetermined range during theperiod of an acoustic pulse.

16. The acoustic pulse generating apparatus of claim 15 wherein saidacoustic pistons are connected to travel in opposite directions, theaxes of travel of said acoustic pistons being colinear so that there isno net force or torque on the apparatus as a result of the operation ofsaid acoustic pistons.

17. The acoustic pulse generating apparatus of claim 16 furthercomprising a watertight capsule enclosing said mechanically resonantstructure, said drive means, and said means for changing the naturalfrequency of oscillatory of said mechanically resonant structure so thatsaid acoustic pulse generating apparatus may be operated at a depthbelow the surface of the water.

18. The acoustic pulse generating apparatus of claim 17 furthercomprising means for compensating the effect of static water pressure onsaid acoustic pistons during the generating of an acoustic pulse.

19. The acoustic pulse generating apparatus of claim 18 wherein saidpressure compensating means comprises an air chamber associated witheach of said acoustic pistons, the inner surface of each of saidacoustic pistons comprising one wall of its associated air chamber, eachof said air chambers containing air under pressure sufficient to balancethe static pressure of the water on the outer faces of said acousticpistons during the generating of an acoustic pulse.

20. The acoustic pulse generating apparatus of claim 19 wherein saidmeans for storing energy in said mechanically resonant structurecomprises a source of high pressure air and means for introducing saidhigh pressure air into each of said chambers before the start of anacoustic pulse to move said acoustic pistons outwardly of their neutralpositions.

21. The acoustic pulse generating apparatus of claim 20 wherein thepressure of the air provided by said source is sufficient to move saidpistons to the outwardmost limits of their strokes.

22. The acoustic pulse generating apparatus of claim 20 wherein saidmeans for releasing the energy stored in said mechanically resonantstructure comprises means associated with each of said air chambers forquickly reducing the air pressure in said air chambers to a level whichsubstantially balances the static pressure of the water on the outerfaces of said pistons.

23. The acoustic pulse generating apparatus of claim 22 wherein saidmeans for quickly reducing the air pressure in said air chamberscomprises a quick release valve connected to each of said air chambersand an air tank connected to each of said quick release valves toreceive air through said quick release valves from said air chambers.

24. The acoustic pulse generating apparatus of claim wherein said drivemeans comprises an hydraulic actuator for imparting oscillatory motionsto said mechanically resonant structure.

25. The acoustic pulse generating apparatus of claim 24 furthercomprising control means responsive to the oscillations of said acousticpistons for controlling said hydraulic actuator.

26. The acoustic pulse generating apparatus of claim 25 wherein saidcontrol means comprises motion sensing means for producing a signal inresponse to the motions of said acoustic pistons, delay means fordelaying said signals from said motion sensing means, and a servocontrol valve for controlling said hydraulic actuator in response tosignals from said delay means.

27. The acoustic pulse generating apparatus of claim 26 wherein saidmotion sensing means comprises acceleration sensing means mounted onsaid acoustic pistons for producing output signals corresponding to theacceleration of said acoustic pistons.

28. The acoustic pulse generating apparatus of claim 27 furthercomprising means responsive to the difference between the magnitude ofsaid signal from said acceleration sensing means and a reference signalfor controlling said servo valve to cause said actuator to apply greaterforce to said mechanically resonant structure when said signal from saidacceleration sensing means is less than said reference signal, and toapply less force to said mechanically resonant structure when themagnitude of said signal from said acceleration sensing means is greaterthan said reference signal.

29. The acoustic pulse generator of claim 1, further comprising meansfor storing energy in said mechanically resonant structure prior to thestart of an acoustic pulse, and means for releasing the energy stored insaid mechanically resonant structure at the start of an acoustic pulseso as to cause said acoustic pulse to start at a high output powerlevel.

1. An acoustic pulse generator for generating acoustic pulses in amedium which acts as a reactive load, each acoustic pulse comprising anacoustic wave having a frequency which varies over a predetermined rangeduring the period of said pulse, said acoustic pulse generatorcomprising: a mechanically resonant structure capable of storingmechanical energy in the form of oscillatory movements; a transducerconnected to said mechanically resonant structure, said transducer beingin contact with said medium to convert the oscillatory movements of saidmechanically resonant structure into acoustic waves in said medium;drive means for imparting sufficient energy to said mechanicallyresonant structure to produce an acoustic pulse; and means for changingthe natural of oscillatory of said mechanically resonant structure overa predetermined range during the period of an acoustic pulse.
 2. Theacoustic pulse generator of claim 1 wherein said drive means comprisesan actuator for imparting oscillatory motions to said mechanicallyresonant structure.
 3. The acoustic pulse generator of claim 2 whereinsaid transducer comprises a piston connected to said mechanicallyresonant structure, the face of said piston being in contact with saidmedium to convert the oscillatory movements of said mechanicallyresonant structure into acoustic waves in said meDium.
 4. The acousticpulse generator of claim 3 further comprising means responsive to themotions of said system for controlling the frequency of operation ofsaid actuator.
 5. The acoustic pulse generator of claim 1 wherein saidmechanically resonant structure comprises a pneumatic spring systemincluding an air cylinder, a piston disposed within said air cylinder sothat when said piston is displaced from a neutral position, adifferential pressure is created which tends to return said piston tosaid neutral position.
 6. The acoustic pulse generator of claim 5further comprising a beam pivotally connected to said piston of saidpneumatic spring system so that the action of said piston issubstantially transverse to said beam, said transducer and said drivemeans being connected to said beam at different points along the lengththereof.
 7. The acoustic pulse generator of claim 6 wherein said meansfor changing the natural frequency of oscillatory of said mechanicallyresonant structure comprises means for changing the ratio of thedistance between the point of connection of said pneumatic spring systemto said beam and the point of connection of said drive means to saidbeam, and the distance between the point of connection of saidtransducer and said beam and the point of connection of said drive meansand said beam.
 8. The acoustic pulse generator of claim 7 wherein saidmeans for changing the natural frequency of oscillatory of saidmechanically resonant structure comprises a ball screw mounted on saidbeam along a portion of the length thereof, a motor for driving saidball screw, and a cross-slide engaging said ball screw, said drive meansbeing connected to said ball screw so that, upon operation of saidmotor, said cross-slide is caused to move along the length of said beam,thereby changing the point of application of said drive means to saidbeam.
 9. The acoustic pulse generator of claim 1 wherein saidmechanically resonant structure comprises an elastic beam capable ofstoring mechanical energy when elastically deformed and releasing saidenergy in the form of oscillatory movements.
 10. The acoustic pulsegenerator of claim 9 wherein said transducer is connected by amechanical linkage to a free end of said elastic beam.
 11. The acousticpulse generator of claim 10 wherein said drive means comprises anactuator for imparting oscillatory motions to said elastic beam.
 12. Theacoustic pulse generator of claim 11 wherein said means for changing thefrequency of oscillatory of said elastic beam comprises a support memberengaging said elastic beam, said support member being stationary in aplane perpendicular to the longitudinal axis of said elastic beam butmovable along the length of said elastic beam to change the naturalfrequency of oscillatory thereof.
 13. The acoustic pulse generator ofclaim 11 wherein said actuator engages said elastic beam atapproximately right angles to the longitudinal axis thereof so as toimpart bending oscillations thereto.
 14. The acoustic pulse generator ofclaim 11 wherein said actuator engages a crank arm extending at rightangles to the longitudinal axis of said elastic beam so as to imparttorsional oscillatory to said elastic beam.
 15. Acoustic pulsegenerating apparatus for generating acoustic pulses in water, eachacoustic pulse comprising an acoustic wave having a frequency whichvaries over a predetermined range during the period of said pulse, saidacoustic pulse generating apparatus comprising: a mechanically resonantstructure capable of storing mechanical energy in the form ofoscillatory movements; a pair of acoustic pistons connected to saidmechanically resonant structure, the faces of said acoustic pistonsbeing in contact with the water to convert the oscillatory movements ofsaid mechanically resonant structure into acoustic waves in the water;drive means for imparting sufficient energy to said mechanicallyresonant structure to produce an acoustic pulse; and means for changingthe natural frequency of oscillatory of said mechanically resonantstructure over a predetermined range during the period of an acousticpulse.
 16. The acoustic pulse generating apparatus of claim 15 whereinsaid acoustic pistons are connected to travel in opposite directions,the axes of travel of said acoustic pistons being colinear so that thereis no net force or torque on the apparatus as a result of the operationof said acoustic pistons.
 17. The acoustic pulse generating apparatus ofclaim 16 further comprising a watertight capsule enclosing saidmechanically resonant structure, said drive means, and said means forchanging the natural frequency of oscillatory of said mechanicallyresonant structure so that said acoustic pulse generating apparatus maybe operated at a depth below the surface of the water.
 18. The acousticpulse generating apparatus of claim 17 further comprising means forcompensating the effect of static water pressure on said acousticpistons during the generating of an acoustic pulse.
 19. The acousticpulse generating apparatus of claim 18 wherein said pressurecompensating means comprises an air chamber associated with each of saidacoustic pistons, the inner surface of each of said acoustic pistonscomprising one wall of its associated air chamber, each of said airchambers containing air under pressure sufficient to balance the staticpressure of the water on the outer faces of said acoustic pistons duringthe generating of an acoustic pulse.
 20. The acoustic pulse generatingapparatus of claim 19 wherein said means for storing energy in saidmechanically resonant structure comprises a source of high pressure airand means for introducing said high pressure air into each of saidchambers before the start of an acoustic pulse to move said acousticpistons outwardly of their neutral positions.
 21. The acoustic pulsegenerating apparatus of claim 20 wherein the pressure of the airprovided by said source is sufficient to move said pistons to theoutwardmost limits of their strokes.
 22. The acoustic pulse generatingapparatus of claim 20 wherein said means for releasing the energy storedin said mechanically resonant structure comprises means associated witheach of said air chambers for quickly reducing the air pressure in saidair chambers to a level which substantially balances the static pressureof the water on the outer faces of said pistons.
 23. The acoustic pulsegenerating apparatus of claim 22 wherein said means for quickly reducingthe air pressure in said air chambers comprises a quick release valveconnected to each of said air chambers and an air tank connected to eachof said quick release valves to receive air through said quick releasevalves from said air chambers.
 24. The acoustic pulse generatingapparatus of claim 15 wherein said drive means comprises an hydraulicactuator for imparting oscillatory motions to said mechanically resonantstructure.
 25. The acoustic pulse generating apparatus of claim 24further comprising control means responsive to the oscillations of saidacoustic pistons for controlling said hydraulic actuator.
 26. Theacoustic pulse generating apparatus of claim 25 wherein said controlmeans comprises motion sensing means for producing a signal in responseto the motions of said acoustic pistons, delay means for delaying saidsignals from said motion sensing means, and a servo control valve forcontrolling said hydraulic actuator in response to signals from saiddelay means.
 27. The acoustic pulse generating apparatus of claim 26wherein said motion sensing means comprises acceleration sensing meansmounted on said acoustic pistons for producing output signalscorresponding to the acceleration of said acoustic pistons.
 28. Theacoustic pulse generating apparatus of claim 27 further comprising meansresponsive to the difference between the magnitude of said signal fromsaid acceleration sensing means and a reference signal for controllingsaid servo vAlve to cause said actuator to apply greater force to saidmechanically resonant structure when said signal from said accelerationsensing means is less than said reference signal, and to apply lessforce to said mechanically resonant structure when the magnitude of saidsignal from said acceleration sensing means is greater than saidreference signal.
 29. The acoustic pulse generator of claim 1, furthercomprising means for storing energy in said mechanically resonantstructure prior to the start of an acoustic pulse, and means forreleasing the energy stored in said mechanically resonant structure atthe start of an acoustic pulse so as to cause said acoustic pulse tostart at a high output power level.